This invention relates to chopper type switching circuits, and more particularly a chopper type switching circuit especially suited as the switching circuit of an FM MPX stereophonic signal demodulation circuit.
For a phase comparator or a synchronous detector in a PLL (phase locked loop) type MPX (multiplex) stereophonic signal demodulation circuit, a so-called "chopper type switching circuit" is conventionally employed. This circuit employs a pair of transistor devices alternatively turned on and off by using a signal synchronous with a subcarrier signal or a pilot signal and a signal opposite in phase to the aforementioned signal.
A switching circuit of this type, as shown in FIG. 1, comprises: a first series circuit of resistors R1 and R2; a second series circuit of resistors R3 and R4; a switching element, or a transistor Q1, having a control terminal (base) and having its input and output terminals connected between the series connection point of the resistors R1 and R2 and a reference voltage (Vc) line. A switching element, or a transistor, Q2 is similarly connected between the series connection point of the resistors R3 and R4 and the reference voltage (Vc) line. An input signal IN to be chopped is applied to the common input terminal of the first and second series circuits, and the chopped output is applied to output terminals OUT1 and OUT2 by alternately rendering the transistors Q1 and Q2 conductive and non-conductive. Accordingly, a signal in phase with a control signal applied to the base of the transistors and a signal opposite in phase to the aforementioned signal are separately provided at the output terminals OUT1 and OUT2. In FIG. 1, reference characters R8 and R9 designate the base input resistances, and RL1 and RL2, the loads.
If in the circuit thus organized the transistor Q1 is conductive while the transistor Q2 is nonconductive under the condition no input signal is available, then the voltage V1 at the output terminal OUT1 is: ##EQU1## When the transistor Q1 is nonconductive while the transistor Q2 is conductive, the voltage V1' at the output terminal OUT1 is: ##EQU2## Accordingly, the leakage component of a switching signal has the following difference between these voltages, that is, the peak value is provided at the output terminal OUT1. Since the higher input impedance is preferable in the chopper circuit, the resistors R1 and R3 are high in resistance. Therefore, the leakage of the switching signal is significant.
This phenomenon also occurs with the output terminal OUT2.